More Evidence Points To Impact As Dinosaur Killer

MORE EVIDENCE POINTS TO IMPACT AS DINOSAUR KILLER

Two new impact crater sites in Belize and Mexico add further evidence to the hypothesis that an asteroid or comet collided with Earth about 65 million years ago, subsequently killing off the dinosaurs and many other species on the planet.

Researchers Adriana Ocampo of NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA, and Kevin Pope of Geo Eco Arc Research, La Canada-Flintridge, CA, led an international team that discovered the two new sites during a recent expedition sponsored by NASA's Exobiology Program and The Planetary Society, Pasadena, CA.

"We discovered an important new site in Alvaro Obregon, Mexico, about 140 miles (230 kilometers) from the rim of the Chicxulub crater. This crater was formed when a 6-to-8-mile diameter (10-to-14-kilometer diameter) asteroid or comet collided with Earth," Ocampo said.

"The site contains two layers of material, or ejecta, thrown out by the impact that flowed across the surface like a thick fluid, known as fluidized ejecta lobes," added Pope. "This is the closest surface exposure of ejecta to the Chicxulub crater that has yet been found and the best example known on Earth from a really big impact crater."

Centered on the coast of Yucatan, Mexico, the Chicxulub crater is estimated to be about 120 miles (200 kilometers) in diameter. The impact 65 million years ago kicked up a global cloud of dust and sulfur gases that blocked sunlight from penetrating through the atmosphere and sent Earth into a decade of near-freezing temperatures. The drop in temperature and related environmental effects are thought to have brought about the demise of the dinosaurs and about 75 percent of the other species on Earth.

The Earth orbits the Sun in a swarm of so-called near-Earth objects, whether they are comets or asteroids, yet the science of detecting and tracking them is still relatively young. Only a handful of astronomers around the world search for these objects, and they estimate that currently only about one-tenth of the population of near-Earth objects has been detected. Chicxulub is the only impact event that has been correlated with mass extinctions to date. The site has been dated geologically to the boundary between the Cretaceous and Tertiary periods, also known as the K/T boundary.

Local geologist Brian Holland of Punta Gorda, Belize, guided the expedition to another new ejecta site about 290 miles (480 kilometers) from the crater rim. This Belize site contains tiny spheres of altered green glass, called tektites. Tektites are rocks that have been melted to glass by the severe heat of an impact. Expedition member Jan Smit of Free University, Amsterdam, noted that the Belize tektites were similar to those found in Haiti and northern Mexico. This finding links the stratigraphy of the Belize sites to the more distant Caribbean and Mexican ejecta sites.

Alfred Fischer of the University of Southern California, Michael Gibson of the University of Tennessee at Martin, and Jaime Urrutia and Francisco Vega of the National Autonomous University of Mexico helped the team collect 900 pounds (400 kilograms) of samples, including drill cores, for paleomagnetic studies. They also collected fossils from the site to help date the deposits and add new pieces to the puzzle of what happened at Chicxulub 65 million years ago.

Impact ejecta is very rare on Earth, but covers much of the surface of Mars because Mars' surface has remained stable and unchanged for billions of years, thus preserving debris from these rare impact events. Also, such fluidized ejecta lobes have never been observed directly on Earth before and can serve as an excellent laboratory for studying the ejecta lobes surrounding many Martian craters.

"The discovery of these new ejecta sites is very exciting," said team co-leader Ocampo. "It is like seeing a bit of Mars on Earth."

The exact nature of these ejecta lobes on Mars remains a mystery, Ocampo noted. Some scientists think they were created by an abundance of water in the Martian crust, which turned the ejecta into a muddy, molasses-like material. Others suggest the fluidized ejecta lobes were enabled by a much thicker atmosphere in Mars' early history. As flying ejecta from an impact event flew through the Martian atmosphere, it was reduced by friction to a very dense, turbulent cloud of debris, which also flowed like water. Study of the Chicxulub fluidized ejecta may help settle this debate and shed new light on theories that the Martian surface may once have been more hospitable for life.

Volunteers who assisted The Planetary Society and the scientists in the field have posted their photographs of the expedition on The Planetary Society web site at the following URL:

Ocampo and Pope's research was funded in part by the Exobiology Program of NASA's Office of Space Science, Washington, DC. NASA's Jet Propulsion Laboratory is a division of the California Institute of Technology, Pasadena, CA.